Every summer when it get hot out there, I've been experiencing gasoline over flow from gas cap every after i pump the gas full. Why??

Most Z gas caps leak when you take a corner or accelerate hard just after you fill the tank. In your case it may not be related to the hot weather? I usually just put 3/4 or so in mine, just to be safe (although it does not leak very much when I fill it)

It is unusually hot in Seattle last few days, just don't fill the tank all the way.

Gas really expands whan you park outside in this heat & if the tank is full, forces it out the filler.

Put the pump filler hose fully into the car's rubber hose, when the pump shuts off (sensing fuel has reached the tip), let it sit a bit to let the bubbling stop, then one more squeeze of the nozzle. When it stops this time, you're as chock full as you want to be, and even this may be a bit "over" full.

This of course presumes that your evap tank and hoses are in good condition. If they're not, then you'll have other problems.

2¢

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A local Hess station does not have the fume-capture hose on the nozzle. For that reason (and the dedicated Super pump) I only purchase from that station.

I too only keep the car about 1/2 to 3/4 fuel. This way I know I always have fresh fuel.

I'm not referring to the rubber boot to capture fumes, I'm referring to the overflow sensor which, AFAIK, goes back to the 70's, even before unleaded became the norm for gasoline.

As such, a gasoline station that does NOT have the overflow sensor... well I recall in Iowa seeing the gas barrel with the simple on/off valve that COULD be left to drain the tank, but I've NEVER seen a gas station without an overflow check valve on their pumps. In fact, I believe that's a requirement for a gas pump.

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I'm not referring to the rubber boot to capture fumes, I'm referring to the overflow sensor which, AFAIK, goes back to the 70's, even before unleaded became the norm for gasoline.

As such, a gasoline station that does NOT have the overflow sensor... well I recall in Iowa seeing the gas barrel with the simple on/off valve that COULD be left to drain the tank, but I've NEVER seen a gas station without an overflow check valve on their pumps. In fact, I believe that's a requirement for a gas pump.

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All gas pumps are mandated to include the automatic shut-off by the EPA.